1. Field of the Invention
The present invention relates to a roll forming method and a roll forming device for draw forming sheet metal materials
2. Description of the Related Art
A need exists for speeding up forming processes, in view of cost-cutting and, in order to realize the need, a continuous roll forming process preferably replaces an intermittent press forming process. However, in the case of prior art roll forming, as shown in FIG. 7, as each outer surface of rotating upper and lower rolls (punch roll 100 and die roll 200) forms a cylindrically curved surface, both roll 100 and roll 200, and a sheet metal material 300 are allowed to make a line contact, but are not allowed to make a plane to plane contact as is performed by using pressing (blank holding) pads for preventing wrinkles in the press forming. Meanwhile, the punch roll 100 is provided with a punch 110 corresponding to a drawn shape and the die roll 200 is provided with a corresponding concaved die 210 engaging the punch 110.
For the above reason, while the draw forming is performed on a thin metal material having a wall thickness less than 0.20 mm by the roll forming, due to a small stiffness of the thin material, it is very difficult to get satisfactory performance of the draw forming in the same number of steps as that of the steps in a press forming process. That is to say, as fewer steps in the roll forming process cause, as shown in the FIG. 8 and FIG. 9A, an excessive drawn strain of a material 300 (drawn displacement d0) in the course of the forming process, a swelling 311 is produced at the central part of a formed portion 310. FIG. 9A is a plan view of a process of roll forming and FIG. 9B is a perspective view of process of roll forming. In consequence, at both sides 320 (flange areas) of the drawn portion 310, wrinkles 330 are produced and result in folds 340 (ref. to FIG. 9C, FIG. 9D, FIG. 9E). FIG. 9C is a plan view of after a process of the roll forming and FIG. 9D is a magnified view of portion E in FIG. 9C after the process of roll forming and FIG. 9E is a perspective view after the process of roll forming. It is thus necessary to perform the draw forming step by step to avoid producing wrinkles 330. The number of steps in the roll forming process is more than double compared to that of the steps in the press forming process.
On the other hand, a roll forming apparatus as described, for example, in U.S. Pat. No. 6,339,947 is known and described later, this roll forming apparatus has a punch on a flat platen segment which rotates and changes the direction of motion by a cam mechanism and can prevent production of wrinkles.
The roll of forming apparatus as described in the above Patent, however, has a very complicated mechanism and accordingly incurs a substantial increase in cost.